Coaxial transmitter optical subassembly (tosa) with cuboid type to laser package and optical transceiver including same

ABSTRACT

A coaxial transmitter optical subassembly (TOSA) including a cuboid type TO laser package may be used in an optical transceiver for transmitting an optical signal at a channel wavelength. The cuboid type TO laser package is made of a thermally conductive material and has substantially flat outer surfaces that may be thermally coupled to substantially flat outer surfaces on a transceiver housing and/or on other cuboid type TO laser packages. An optical transceiver may include multiple coaxial TOSAs with the cuboid type TO laser packages stacked in the transceiver housing. The cuboid type TO laser package may thus provide improved thermal characteristics and a reduced size within the optical transceiver.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 13/760,533 filed Feb. 6, 2013, which is fullyincorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to laser packages and more particularly,to a coaxial transmitter optical subassembly (TOSA) with a cuboid typeTO laser package for use in an optical transceiver.

BACKGROUND INFORMATION

Optical transceivers are used to transmit and receive optical signalsfor various applications including, without limitation, internet datacenter, cable TV broadband, and fiber to the home (FTTH) applications.Optical transceivers provide higher speeds and bandwidth over longerdistances, for example, as compared to transmission over copper cables.The desire to provide higher speeds in smaller optical transceivermodules for a lower cost has presented challenges, for example, withrespect to thermal management, insertion loss, and manufacturing yield.

Optical transceiver modules generally include one or more laser diodepackages for housing a laser diode and for providing electricalconnections and optical couplings to the laser diode. One challenge withoptical transceiver modules is providing thermal management, especiallywith new optical transceiver modules that are designed to provide higherdata rates within a relatively small form factor. In particular, theheat generated by the laser diode and associated components in the laserpackage may adversely affect the laser wavelengths or potentially evendamage the laser and/or other components. Conventional TO can laserpackages do not provide good heat conduction because the roundedsurfaces of the TO can package do not provide sufficient contact forthermal coupling with other surfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages will be better understood byreading the following detailed description, taken together with thedrawings wherein:

FIGS. 1A and 1B are functional block diagrams of multiple channeloptical transceivers, consistent with embodiments of the presentdisclosure.

FIG. 2 is a perspective view of an embodiment of an optical transceivermodule including coaxial TOSAs with cuboid-type TO laser packages.

FIGS. 3A and 3B are top and bottom perspective views, respectively, ofanother embodiment of an optical transceiver module including coaxialTOSAs with cuboid-type TO laser packages.

FIG. 4 is a perspective view of an embodiment of the coaxial TOSA foruse in the optical transceiver modules shown in FIGS. 2, 3A, and 3B.

FIG. 5 is a perspective view of an embodiment of the cuboid type TOlaser package used in the mini coaxial TOSA shown in FIG. 4.

FIG. 6 is a perspective view of another embodiment of a coaxial TOSAwith a cuboid type TO laser package for use in an optical transceivermodule.

FIG. 7 is a perspective view of another embodiment of the cuboid type TOlaser package used in the mini coaxial TOSA shown in FIG. 6.

DETAILED DESCRIPTION

A coaxial transmitter optical subassembly (TOSA) including a cuboid typeTO laser package, consistent with embodiments of the present disclosure,may be used in an optical transceiver for transmitting an optical signalat a channel wavelength. The cuboid type TO laser package is made of athermally conductive material and has substantially flat outer surfacesthat may be thermally coupled to substantially flat outer surfaces on atransceiver housing and/or on other cuboid type TO laser packages. Anoptical transceiver may include multiple coaxial TOSAs with the cuboidtype TO laser packages stacked in the transceiver housing. The cuboidtype TO laser package may thus provide improved thermal characteristicsand a reduced size within the optical transceiver.

As used herein, “cuboid type TO package” refers to a laser packagestructure having a generally cuboid or parallelepiped outer shape formedby at least three substantially flat and orthogonal outer surfaces. Forclarification, the term “TO” or “transistor outline” is derived from areference to the traditional cylindrical package or “can” thathistorically encased a transistor, but as used herein, is otherwiseunrelated to such transistor package. As used herein, “channelwavelengths” refer to the wavelengths associated with optical channelsand may include a specified wavelength band around a center wavelength.In one example, the channel wavelengths may be defined by anInternational Telecommunication (ITU) standard such as the ITU-T densewavelength division multiplexing (DWDM) grid. The term “coupled” as usedherein refers to any connection, coupling, link or the like and“optically coupled” refers to coupling such that light from one elementis imparted to another element. Such “coupled” devices are notnecessarily directly connected to one another and may be separated byintermediate components or devices that may manipulate or modify suchsignals.

As used herein, “thermally coupled” refers to a direct or indirectconnection or contact between two components resulting in heat beingconducted from one component to the other component and “thermallyisolated” refers to an arrangement where heat is prevented from beingconducted to the isolated component from an external environment. In athermally isolated multi-channel TOSA, for example, heat external to theTOSA is prevented from being conducted to one or more components in theTOSA. As used herein, “thermally shielded” refers to an arrangement thatprevents heat from being transferred by convection or radiation to theshielded component. Thermally isolated and thermally shielded do notnecessarily require an arrangement to prevent all heat from beingconducted or transferred.

Referring to FIG. 1A, an optical transceiver 100, consistent withembodiments of the present disclosure, is shown and described. In thisembodiment, the optical transceiver 100 transmits and receives four (4)channels using four different channel wavelengths (λ₁, λ₂, λ3, λ4) andmay be capable of transmission rates of at least about 10 Gbps perchannel. In one example, the channel wavelengths λ₁, λ₂, λ₃, λ₄ may be1270 nm, 1290 nm, 1080 nm, and 1330 nm, respectively. The opticaltransceiver 100 may also be capable of transmission distances of 2 km toat least about 10 km. The optical transceiver 100 may be used, forexample, in internet data center applications or fiber to the home(FTTH) applications.

This embodiment of the optical transceiver 100 includes multipletransmitter optical subassemblies (TOSAs) 120 a-d for transmittingoptical signals on different channel wavelengths and a multi-channelreceiver optical subassembly (ROSA) 130 for receiving optical signals ondifferent channel wavelengths. The TOSAs 120 a-d and the multi-channelROSA 130 are located in a transceiver housing 102. A transmit connectingcircuit 104 and a receive connecting circuit 108 provide electricalconnections to the TOSAs 120 a-d and the multi-channel ROSA 130,respectively, within the housing 102. The transmit connecting circuit104 is electrically connected to the electronic components (e.g., thelaser, monitor photodiode, etc.) in each of the TOSAs 120 a-d and thereceive connecting circuit 108 is electrically connected to theelectronic components (e.g., the photodiodes, the TIA, etc.) in themulti-channel ROSA 130. The transmit connecting circuit 104 and thereceive connecting circuit 108 include at least conductive paths toprovide electrical connections and may also include additionalcircuitry.

A multi-fiber push on (MPO) connector 110 provides optical connectionsto the TOSAs 120 a-d and the multi-channel ROSA 130 within the housing102. The MPO connector 110 is optically coupled to the TOSAs 120 a-d andthe multi-channel ROSA 130 via transmit optical fibers 122 and receiveoptical fibers 132, respectively. The MPO connector 110 is configured tobe coupled to a mating MPO connector 112 such that the optical fibers122, 132 in the optical transceiver 100 are optically coupled toexternal optical fibers 114.

Each of the TOSAs 120 a-d may be a coaxial TOSA with a coaxialconfiguration electrically connected at one end to conductive paths onthe transmit connecting circuit 104 and optically coupled at the otherend to a respective one of the optical fibers 122. Each of the TOSAs 120a-d may include a laser for generating laser light at the assignedchannel wavelength and optics for coupling the laser light into therespective optical fiber 122. The lasers in the TOSAs 120 a-d thusconvert electrical data signals (TX_D1 to TX_D4) received via thetransmit connecting circuit 104 into modulated optical signalstransmitted over transmit optical fibers 122. The lasers may include,for example, distributed feedback (DFB) lasers with diffractiongratings. Each of the TOSAs 120 a-d may also include a monitorphotodiode for monitoring the light emitted by the lasers. Each of theTOSAs 120 a-d may further include one or more temperature controldevices, such as a resistive heater and/or a thermoelectric cooler(TEC), for controlling a temperature of the lasers, for example, tocontrol or stabilize the laser wavelengths.

The multi-channel ROSA 130 includes a photodetector array 134 including,for example, photodiodes optically coupled to a fiber array 133 formedby the ends of the receive optical fibers 132. The multi-channel ROSA130 also includes a multi-channel transimpedance amplifier 136electrically connected to the photodetector array 134. The photodetectorarray 134 and the transimpedance amplifier 136 detect and convertoptical signals received from the fiber array 133 into electrical datasignals (RX_D1 to RX_D4) that are output via the receive connectingcircuit 108.

This embodiment of the optical transceiver 100 does not include anoptical multiplexer or demultiplexer. The optical signals may bemultiplexed and demultiplexed external to the optical transceiver 100.

Referring to FIG. 1B, another embodiment of an optical transceiver 100′includes the same light engine (e.g., TOSAs 120 a-d and ROSA 130)described above together with an optical multiplexer 111 and an opticaldemultiplexer 113. The optical multiplexer 111 and the opticaldemultiplexer 113 both may include arrayed waveguide gratings (AWGs).The optical multiplexer 111 is optically coupled to the transmit opticalfibers 122 and the optical demultiplexer 113 is optically coupled to thereceive optical fibers 132. The optical multiplexer 111 multiplexes theoptical signals being transmitted over transmit optical fibers 122 toprovide a multiplexed optical signal on an output optical fiber 115. Theoptical demultiplexer 113 demultiplexes a multiplexed optical signalreceived on an input optical fiber 117 to provide received opticalsignals on receive optical fibers 132. The output optical fiber 115 andthe input optical fiber 117 are coupled to an output optical connector116 and an input optical connector 118, respectively.

This embodiment of the optical transceiver 100′ includes 4 channels andmay be configured for coarse wavelength division multiplexing (CWDM),although other numbers of channels are possible. This embodiment of theoptical transceiver 100′ may also be capable of transmission rates of atleast about 10 Gbps per channel and transmission distances of 2 km to atleast about 10 km and may be used in internet data center applicationsor fiber to the home (FTTH) applications.

Referring to FIG. 2, an embodiment of an optical transceiver module 200with an MPO connector 210 is described and shown in greater detail. Theoptical transceiver module 200 may be designed to have a relativelysmall form factor with minimal space. The optical transceiver module 200includes a transceiver housing 202, four coaxial TOSAs 220 stackedtogether in one region of the housing 202, and a multi-channel ROSA 230located in another region of the housing 202. The coaxial TOSAs 220 areelectrically connected to transmit flexible printed circuits (FPCs) 204at one end of the housing 202 and optically coupled to the MPO connector210 at the other end of the housing 202 via transmit optical fibers 222.The multi-channel ROSA 230 is electrically connected to a receiveflexible printed circuit (FPC) 208 at one end of the housing 202 andoptically coupled to the MPO connector 210 at the other end of thehousing 202 via receive optical fibers 232.

Each of the coaxial TOSAs 220 includes a cuboid type TO laser package250 that contains a laser submount 226, a diode laser 227 on thesubmount 226, and a lens 223. The laser submount 226 electricallyconnects the diode laser 227 to the respective transmit FPC 204, forexample, using wire bonding. The lens 223 optically couples the laser227 to the respective transmit optical fiber 222. The cuboid type TOlaser package 250 has a generally cuboid or parallelepiped outer shapeto provide heat dissipation and/or thermal shielding, as will bedescribed in greater detail below. Each of the coaxial TOSAs 220 has acoaxial configuration such that electrical connections are made from oneend of the TOSA 220 and an optical coupling is made from the other endof the TOSA 220.

The multi-channel ROSA 230 includes a fiber array 233 optically coupledto a photodetector array 234 and a transimpedance amplifier (TIA) 236electrically connected to the photodetector array 234. The end faces ofthe optical fibers 232 in the fiber array 233 may be angled (e.g., at45°) such that the light is reflected from the angled face to couplewith the respective photodiodes in the photodetector array 234. The TIA236 is electrically connected to the receive FPC 208, for example, usingwire bonding.

Referring to FIGS. 3A and 3B, another embodiment of an opticaltransceiver module 200′ including an optical multiplexer and an opticaldemultiplexer is shown in greater detail. The optical transceiver module200′ includes the coaxial TOSAs 220, the multi-channel ROSA 230, and theFPCs 204, 208, as described above. This embodiment of the opticaltransceiver module 200′ further includes an AWG housing portion 202 athat contains a multiplexing AWG 211 and a demultiplexing AWG 213. TheAWG housing portion 202 a may be coupled to and/or extend from thetransceiver housing 202. The multiplexing AWG 211 is optically coupledto the coaxial TOSAs 220 via transmit optical fibers 222 and thedemultiplexing AWG 213 is optically coupled to the ROSA 230 via thereceive optical fibers 232. The multiplexing AWG 211 and thedemultiplexing AWG 213 are optically coupled to output optical connector216 and input optical connector 218, respectively, via output opticalfiber 215 and input optical fiber 217, respectively.

These embodiments of the optical transceiver module 200, 200′ bothinclude coaxial TOSAs 220 with cuboid type TO packages, as will bedescribed in greater detail below. The coaxial TOSA 220 with the cuboidtype TO package may also be used in other types of optical transceiverssuch as the multi-channel transceiver used in an optical line terminal(OLT), as described in greater detail in U.S. Patent ApplicationPublication No. 2014/0161459, which is fully incorporated herein byreference. The coaxial TOSA 220 with the cuboid type TO package may alsobe used in an optical transmitter without a ROSA.

As shown in greater detail in FIGS. 4 and 5, each coaxial TOSA 220includes a cuboid type TO laser package 250 that contains the lasersubmount 226, the diode laser 227, and the lens 223 and/or other optics.The cuboid type TO laser package 250 includes an electrical connectingend 252 opposite an optical coupling end 254. The laser submount 226 ismounted proximate the electrical connecting end 252 such that electricalleads or wires (not shown) may be electrically connected to conductivepaths 229 on the submount and extend from the electrical connecting end252. An optical coupling receptacle 221 extends from the opticalcoupling end 254 for optically coupling the laser 227 to an opticalfiber 222. The conductive paths 229 (and electrical leads), the laser227, the lens 223, the optical coupling receptacle 221 and the opticalfiber 222 are generally aligned or positioned coaxially along alongitudinal axis 2, thereby providing the coaxial configuration of thecoaxial TOSA 220.

A monitor photodiode 228 may also be mounted on the submount 226, forexample, to monitor light emitted from the diode laser 227. In otherembodiments, one or more temperature control devices may be providedwithin or on the cuboid type TO laser package 250. The temperaturecontrol devices may include a heater, such as a resistive heater,located adjacent the diode laser 227 to provide independent control ofthe temperature of the diode laser 227 and thus the wavelength of theemitted laser light. The cuboid type TO laser package 250 facilitatesthis independent temperature control of each laser 227 by preventingheat from outside of the package 250 from affecting the laser 227.Additionally or alternatively, the temperature control device mayinclude a micro thermoelectric cooler (TEC) within the cuboid type TOlaser package 250 to provide the individual and independent temperaturecontrol of the laser 227. A TEC may also be used outside of the cuboidtype TO laser package 250 by thermally coupling to an outside surface ofthe cuboid type TO laser package 250.

The cuboid type TO laser package 250 includes at least one substantiallyflat outer surface substantially orthogonal to the electrical connectingend 252 and the optical coupling end 254 for contacting anothersubstantially flat surface to facilitate heat transfer. In theillustrated embodiment, top, bottom, and side surfaces 256 a-d aresubstantially flat, which allows multiple cuboid type TO laser packages250 to be stacked in a transceiver housing, for example, as shown inFIGS. 2, 3A, and 3B. In this embodiment, the bottom surface 256 bprovides the greatest surface area for heat transfer.

In the illustrated embodiment, the cuboid type TO laser package 250includes first and second side walls 251 extending from a base 253 todefine a compartment 255 (see FIG. 5). The laser submount 226 is locatedin the compartment 255 between the side walls 251. Thus, the laser diode227 is thermally shielded by the side walls 251. This embodiment of thecuboid type TO laser package 250 further includes an end wall 257extending from the base 253 at the optical coupling end 254. The endwall 257 defines an aperture 258 that allows laser light to pass throughfor coupling into the optical fiber 222. Optics, such as an opticalisolator, may also be located within the aperture 258.

The cuboid type TO laser package 250 may be formed as one piece or asmultiple pieces attached together (e.g., the walls 251, 257 attached tothe base 253). Although the illustrated embodiment shows the cuboid typeTO laser package 250 with a particular shape, other shapes andconfigurations are also possible. In other embodiments, for example, thecuboid type TO laser package 250 may be closed at the top (e.g., theside opposite the base 253).

The cuboid type TO laser package 250 may be made of a thermallyconductive material having a thermal conductivity greater than 60W/(m·K) and more specifically greater than 80 W/(m·K) and, for example,about 160 W/(m·K). The cuboid type TO laser package 250 may be made, forexample, from copper tungsten and may also be gold plated, for example,to facilitate soldering. In some embodiments, the cuboid type TO laserpackage 250 may be made from a nickel-cobalt ferrous alloy such as thetype sold under the trademark KOVAR. Other thermally conductivematerials may also be used.

The flat surfaces of the cuboid type TO laser package 250 advantageouslyprovide for increased surface area contact between the package 250 andother packages or the transceiver housing. This increased surface areacontact improves thermal transfer or heat conduction, and thusfacilitates heat dissipation even in a smaller package design comparedto traditional cylindrical type TO packages. In a conventionalcylindrical type TO can package, the flat surfaces at the ends of thepackage cannot effectively be used for thermal transfer because it wouldinterfere with the electrical connections and optical couplings made atthese ends of the cylindrical type TO can package. The cuboid type TOpackage 250 provides the electrical connections and optical couplings atthe ends in a coaxial configuration while also providing flat surfacesfor thermal coupling and for stacking in a compact arrangement.

As shown in FIG. 5, the cuboid type TO laser package 250 may have arelatively small size. In some embodiments, the long axis of the base253 may be less than 3.5 mm (in the illustrated example 3.4 mm). In someembodiments, the long axis of the walls 251 and the spacing between theoutside surfaces of the walls may be less than 2.5 mm (in theillustrated example 2.1 mm). Thus, the cuboid type TO laser package 250may provide a header of about 2 mm square, which is significantlysmaller than a 5.6 mm header of a conventional cylindrical type TO canpackage. Although the walls 251 are shown as having the same size, thisis not a limitation of the present disclosure.

Referring to FIGS. 6 and 7, another embodiment of a cuboid type TO laserpackage 250′ is shown and described. In this embodiment, the cuboid typeTO laser package 250′ includes additional side walls 259 proximate theoptical coupling end 254, instead of the end wall 257. The lens 223 ismounted between the additional side walls 259. In this embodiment, theside walls 251, 259 on each side are separated by a space. In otherembodiments, a cuboid type TO laser package may include a single wall oneach side of the base 253 and extending along at least a portion of thebase 253 or along the entire side of the base 253. In anotherembodiment, a cuboid type TO laser package may include a single wall oneach side of the base 253 and an end wall with an aperture to allowlaser light to pass through. Various other configurations for the cuboidtype TO laser package are within the scope of the present disclosure.

Accordingly, a cuboid type TO laser package, consistent with embodimentsdescribed herein, is used in a coaxial TOSA to provide improved thermalcharacteristics and reduced size. Multiple cuboid type TO laser packagesmay be stacked together within a relatively small space in amulti-channel optical transceiver with improved heat dissipation.

Consistent with an embodiment, a coaxial transmitter optical subassembly(TOSA) includes a cuboid type TO laser package including a base and atleast first and second side walls extending from opposite sides of thebase defining a compartment. The cuboid type TO laser package has aplurality of substantially flat outer surfaces, an optical coupling end,and an electrical connecting end opposite the optical coupling end. Thecuboid type TO laser package is made of a thermally conductive material.The coaxial TOSA also includes a laser submount mounted on the base andbetween the first and second side walls. The laser submount includesconductive paths proximate the electrical connecting end for providingelectrical connections. The coaxial TOSA further includes a laser diodemounted on the laser submount and electrically connected to theconductive paths and optics mounted proximate the optical coupling endfor optically coupling the laser to an optical fiber.

Consistent with another embodiment, an optical transceiver moduleincludes a transceiver housing, a plurality of coaxial TOSAs located inthe transceiver housing for transmitting optical signals at differentchannel wavelengths, and a multi-channel receiver optical subassemblylocated in the transceiver housing for receiving optical signals atdifferent channel wavelengths. Each of the coaxial TOSAs includes acuboid type TO laser package including a base and at least first andsecond side walls extending from opposite sides of the base defining acompartment. The cuboid type TO laser package has a plurality ofsubstantially flat outer surfaces, an optical coupling end, and anelectrical connecting end opposite the optical coupling end. The cuboidtype TO laser package is made of a thermally conductive material. Thecoaxial TOSA also includes a laser submount mounted on the base andbetween the first and second side walls. The laser submount includesconductive paths proximate the electrical connecting end for providingelectrical connections. The coaxial TOSA further includes a laser diodemounted on the laser submount and electrically connected to theconductive paths and optics mounted proximate the optical coupling endfor optically coupling the laser to an optical fiber.

While the principles of the invention have been described herein, it isto be understood by those skilled in the art that this description ismade only by way of example and not as a limitation as to the scope ofthe invention. Other embodiments are contemplated within the scope ofthe present invention in addition to the exemplary embodiments shown anddescribed herein. Modifications and substitutions by one of ordinaryskill in the art are considered to be within the scope of the presentinvention, which is not to be limited except by the following claims.

What is claimed is:
 1. A coaxial transmitter optical subassembly (TOSA)comprising: a cuboid type TO laser package including a base and at leastfirst and second side walls extending from opposite sides of the basedefining a compartment, the cuboid type TO laser package having aplurality of substantially flat outer surfaces, the cuboid type TO laserpackage having an optical coupling end and an electrical connecting endopposite the optical coupling end, wherein the cuboid type TO laserpackage is made of a thermally conductive material; and a laser submountmounted on the base and between the first and second side walls, thelaser submount including conductive paths proximate the electricalconnecting end for providing electrical connections; a laser diodemounted on the laser submount and electrically connected to theconductive paths; and optics mounted proximate the optical coupling endfor optically coupling the laser to an optical fiber.
 2. The coaxialTOSA of claim 1 wherein the cuboid type TO laser package furtherincludes an end wall extending from the base at the optical couplingend, the end wall including an aperture configured to allow laser lightto pass through, wherein the laser diode and the optics are aligned withthe aperture.
 3. The coaxial TOSA of claim 2 wherein the optics includean isolator located inside the aperture and a lens aligned with theaperture.
 4. The coaxial TOSA of claim 1 wherein the cuboid type TOlaser package further includes third and fourth side walls extendingfrom opposite sides of the base proximate the optical coupling end,wherein the optics include a lens located between the third and fourthside walls.
 5. The coaxial TOSA of claim 1 wherein said thermallyconductive material is a nickel-cobalt ferrous alloy.
 6. The coaxialTOSA of claim 1 wherein said thermally conductive material has a thermalconductivity greater than 80 W/(m·K).
 7. The coaxial TOSA of claim 1wherein the optics include a lens.
 8. The coaxial TOSA of claim 1further including a monitor photodiode mounted on the laser submount. 9.The coaxial TOSA of claim 1 wherein a long axis of the base is less than3.5 mm and a long axis of the first and second side walls is less than2.5 mm.
 10. The coaxial TOSA of claim 1 wherein at least one of thesubstantially flat outer surfaces is orthogonal to the electricalconnecting end and the optical coupling end.
 11. An optical transceivermodule comprising: a transceiver housing; a plurality of coaxial TOSAslocated in the transceiver housing for transmitting optical signals atdifferent channel wavelengths, each of the plurality of coaxial TOSAscomprising: a cuboid type TO laser package including a base and at leastfirst and second side walls extending from opposite sides of the basedefining a compartment, the cuboid type TO laser package having aplurality of substantially flat outer surfaces, the cuboid type TO laserpackage having an optical coupling end and an electrical connecting endopposite the optical coupling end, wherein the cuboid type TO laserpackage is made of a thermally conductive material; a laser submountmounted on the base and between the first and second side walls, thelaser submount including conductive paths proximate the electricalconnecting end for providing electrical connections; a laser diodemounted on the laser submount and electrically connected to theconductive paths; and optics mounted proximate the optical coupling endfor optically coupling the laser to an optical fiber; and amulti-channel receiver optical subassembly located in the transceiverhousing for receiving optical signals at different channel wavelengths.12. The optical transceiver of claim 11 further comprising a transmitconnecting circuit electrically connected to the coaxial TOSAs and areceive connecting circuit electrically connected to the ROSA.
 13. Theoptical transceiver of claim 12 further comprising a multi-fiber push on(MPO) connector optically coupled to the coaxial TOSAs and the ROSA. 14.The optical transceiver of claim 12 further comprising an opticalmultiplexer optically coupled to the coaxial TOSAs for multiplexing thetransmitted optical signals into a transmitted multiplexed opticalsignal and an optical demultiplexer coupled to the ROSA fordemultiplexing a received multiplexed optical signal into the receivedoptical signals.
 15. The optical transceiver of claim 11 wherein thecuboid type TO laser packages are stacked against each other such thatat least one substantially flat surface of each of the cuboid type TOlaser packages is thermally coupled to at least one substantially flatsurface of another of the cuboid type TO laser packages.
 16. The opticaltransceiver of claim 11 wherein at least one of the substantially flatsurfaces of at least one of the cuboid type TO laser packages contacts asubstantially flat surface of the transceiver housing.
 17. The opticaltransceiver of claim 11 wherein the cuboid type TO laser package furtherincludes an end wall extending from the base at the optical couplingend, the end wall including an aperture configured to allow laser lightto pass through, wherein the laser diode and the optics are aligned withthe aperture.
 18. The optical transceiver of claim 17 wherein the opticsinclude an isolator located inside the aperture and a lens aligned withthe aperture.
 19. The optical transceiver of claim 11 wherein a longaxis of the base is less than 3.5 mm and a long axis of the first andsecond side walls is less than 2.5 mm.
 20. The optical transceiver ofclaim 11 wherein the plurality of coaxial TOSAs includes four pluralityof coaxial TOSAs configured to transmit at four different channelwavelengths at transmission rates of at least about 10 Gbps per channeland transmission distances of 2 km to at least about 10 km.